1. Field of the Invention
The present invention relates to an image display apparatus for displaying an image with a wide field of view, and is used in the field of providing images such as the field of virtual reality.
2. Description of the Prior Art
Recently, virtual reality providing imaginary images and enhancing the feeling of presence has become remarkably wide-spread, and image display apparatuses of various types have been developed for providing images with wide fields of view. As one of these image display apparatuses, there is an image display apparatus that projects an image, displayed on a display device, onto a screen with magnification of the image by a projection optical system. The apparatus can be used while being held by the observer's hands or mounted on the observer's head so that the screen is positioned in front of the observer's eyes. In this type of image display apparatus, it is necessary to apply light to the observer from all parts of the screen. Therefore, a screen having diffusive-reflectivity or retro-reflectivity is used.
In the case when a screen having diffusive-reflectivity is used, since the degree of light diffusion depends on the incident angle of light onto the screen, the quantity of light going from respective positions of the screen to the observer varies, so that the brightness of the image to be provided is apt to become uneven. In addition, since the projected light proceeds from the screen in all directions and the quantity of the light applied to the observer becomes small, it is inevitable that the brightness of the image observed is reduced. Furthermore, when images having parallax are provided to the left and right eyes to enhance the three-dimensional effect, a phenomenon that the light of the image for the left eye enters the right eye and the light for the right eye enters the left eye, that is, cross-talk arises. Therefore, it is necessary to provide special means for avoiding this.
On the other hand, if a screen having retro-reflectivity is used, light entering into any part of the screen is almost reflected toward each incident direction. Therefore, by projecting light from a direction of the observer's eyes toward the screen, it is possible to direct almost all of the projected light from any part of the screen to the observer's eyes. For this reason, it is possible to provide a bright image without uneven brightness and cross-talk. It is also possible to set the images for the left and right eyes to overlap each other on the screen.
However, in the screen having retro-reflectivity, the pupil diameter of the image provided for the observer, that is the diameter of the light beams expressing individual points of the image at the position of the eyes is defined by only the diameter of the exit pupil of a projection optical system and the distance between the screen and the eyes. Therefore, it is difficult to magnify the pupil diameter of the images for facilitating observation, and at the same time to miniaturize and lighten the apparatus so as to make the apparatus suitable to the mode of usage. For example, if the exit pupil of the projection optical system is made larger, the size and the weight of the projection optical system increase. If the distance between the eyes and screen is elongated, the apparatus inevitably becomes large.
In Japanese Laid-Open Patent Application No. H6-75303, a screen is proposed which is provided with a diffusing layer in front of a retro-reflective layer and has retro-reflectivity and diffusivity. If such a screen is used, the disadvantage of retro-reflectivity is compensated, since the light beams expressing individual points of an image are magnified by the diffusing layer. Therefore, it is expected that an image display apparatus which is small-sized and light-weight and provides an image with a large pupil diameter will be realized.
Man's field of view is large in the horizontal (left and right) direction and small in the vertical (up and down) direction. Nevertheless, in the screen described in the above-described patent application, the light beam is magnified isotropically by the diffusing layer. Hence, at least either excessive magnification in the vertical direction or insufficient magnification in the horizontal direction is caused. That is, if the light beam is magnified so as to meet the field of view in the horizontal direction, the image is excessively magnified beyond the field of view in the vertical direction. On the contrary, if the light beam is magnified so as to meet the field of view in the vertical direction, the image is insufficiently magnified below the field of view in the horizontal direction.
If the light beam is excessively magnified, the image provided becomes dark, and futile light without any probability of entering into eyes arises. If the light beam is insufficiently magnified, a situation arises where some part of the light expressing the image does not enter the eyes depending on the direction of the observer's eyes, i.e., the direction of line of sight. This causes eclipse, that is, partial dropouts of the image to be provided.
The phenomenon that the image to be provided becomes dark by excessive magnification of the light beam can be compensated by increasing the quantity of light in the display device displaying the image. However, doing so requires high performance of the display device itself or a light source illuminating the display device. Therefore, manufacturing cost of the apparatus is increased, and in addition, power consumption and running cost is also increased. Furthermore, when images with parallax are provided to the left and right eyes, excessive magnification of light beams results in cross-talk. In the screen described in the above-mentioned patent application, the extent to which the light beam is magnified by the diffusing layer is not considered.